fix: upgrade postcss-loader from 7.1.0 to 7.3.0 (#139)

Snyk has created this PR to upgrade postcss-loader from 7.1.0 to 7.3.0.

See this package in npm:
https://www.npmjs.com/package/postcss-loader

See this project in Snyk:
https://app.snyk.io/org/plan-x/project/43ecce79-408c-4464-831c-06da1d999308?utm_source=github&utm_medium=referral&page=upgrade-pr

Co-authored-by: snyk-bot <snyk-bot@snyk.io>
Co-authored-by: Shawn O'Connor <ocshawn@ocshawn.com>

package.json

@@ -70,7 +70,7 @@
     "postcss-assets": "^6.0.0",
     "postcss-cli": "^10.0.0",
     "postcss-less": "^6.0.0",
-    "postcss-loader": "^7.1.0",
+    "postcss-loader": "^7.3.0",
     "postcss-preset-env": "^7.8.3",
     "react-test-renderer": "^16.8.4"
   },